The invention relates more specifically to a boosted brake for a motor vehicle, of the type which comprises a pneumatic brake booster actuating a master cylinder, of the type in which the booster comprises a rigid casing inside which can move a transverse partition sealably delimiting a front chamber subjected to a first pressure and a rear chamber subjected to a second pressure, of the type in which the booster comprises a moving piston fixed to the moving partition and comprising a front face which can act on a primary piston of the master cylinder by way of a reaction disk which is fixed to a rear end of the primary piston, of the type in which the booster comprises a control rod which moves in the piston selectively as a function of an axial input force exerted forward against a return force exerted on the rod by a return spring, of the type in which the front end of the control rod comprises at least one plunger which is mounted slideably in the piston and is connected to at least one annular seat of a valve which is able to set up in the rear chamber a second pressure which is greater than the first pressure prevailing in the front chamber so as to cause the moving partition to move, of the type in which the booster comprises a feeler, arranged at the front end of the plunger and passing through a bore leading from the piston, which, when the control rod is in a rest position, is arranged at a defined jump distance from the reaction disk and which is able, when the control rod is actuated with an input force whose intensity is greater than a first defined intensity, to come into contact with the reaction disk in such a way as to transmit to the plunger and to the control rod the reaction force of the master cylinder.
Many examples of boosted brakes of this type are known.
In such a boosted brake, the output force of the booster which is applied to the primary piston of the master cylinder depends on the input force applied to the control rod.
Typically, during the application of a braking force with an intensity greater than a defined intensity, a distinction is made between four phases followed by the output force.
During a first phase referred to as dead travel phase, the output force is substantially zero. This travel corresponds to the travel of the control rod required to overcome the force of the return spring.
Then, during a second phase referred to as jump phase, and for a defined value of the input force, the output force increases abruptly up to a first defined value, independently of the value of the input force. This travel corresponds to the absorption of the jump distance by the feeler.
Then, during a third phase referred to as boost phase, the output force increases substantially linearly from the first defined value up to a second defined value as a function of the input force. This phase corresponds to the boost given to the output force by the booster.
Finally, during a fourth phase referred to as saturation phase, the output force increases substantially linearly from the second defined value as a function of the input force. The increase in the output force is less than the increase during the boost phase, since the moving wall of the booster has reached the end of its travel and provides no more boost. Any additional braking force is provided only by the control rod.
This design presents a problem in terms of progressivity. Specifically, during the second phase, the output force increases abruptly for a defined value of the force exerted on the control rod. The reaction force resulting therefrom also increases abruptly for its part and this results in an unpleasant pedal feel for the driver of the vehicle.
Moreover, the transition between the second and third phases also takes place abruptly for its part and causes the increase in the output force to slow down suddenly, which is manifested at the moment of this transition as a soft feel in the reaction force which is transmitted to the pedal and felt by the driver.